1. Field of the Invention
The present invention relates to a lens barrel incorporating a motor, and more particularly to a lens barrel incorporating a motor provided with a rotor composed of a cylindrical permanent magnet and capable of selecting either a stepping drive in which said cylindrical permanent magnet is step-driven as a stepping motor or a brushless drive in which said magnet is continuously driven as a brushless motor.
2. Description of the Prior Art
In the photo-taking system composed of a camera and lens, a recently promoted automatic feature of the camera is the so-called automatic focusing device, which automatically brings of focusing lens in the lens tube to the in-focus position without manipulation by the user.
The above-mentioned automatic focusing device utilizes the rotary force of a motor for moving the focusing lens to the in-focus position, and an improved hollow sturcture of said motor is disclosed, for example, in the Japanese Patent Laid-open No. 186738/1982 and in the Japanese Utility Model Laid-open No. 28424/1982, in which the rotor is made hollow and the hollow portion is utilized for accommodating the photo-taking or other optical system.
Such a hollow motor can be constructed as a linear motor, a stepping motor, a coreless motor or a brushless motor. The stepping motor has an accurate rotating angle in respons to an input signal and therefore achieves improved focusing accuracy when employed in the control of the focusing lens. However, being designed for stepwise rotation in response to pulse signals, it requires a longer time for moving the focusing lens from an initial position to a final in-focus position. The motor will not be properly driven if a higher driving frequency is employed. Also, the stepping motor has a lower starting torque in comparison with other motors, such as a brushless motor, for driving the focusing lens. To obtain enough starting torque, a larger motor is required, which is inconvenient for incorporating inthe lens tube and which adversely affects the portability of the lens tube, given the motor size and the power source required therefor.
On the other hand, in the case of moving the focusing lens by means of a brushless motor through means for converting the rotary motion into a linear motion, it becomes necessary to precisely control the stopping of the rotor for correctly stopping the focusing lens at the in-focus position, and, for this purpose there is required a complicated control system for controlling the inertia and braking of the rotor.